There is some news regarding the Three Sisters volcano in Oregon, as we now know what the
composition of the magma actively intruding underneath the complex is.
Before I say why this development is important, I will explain the ongoing situation at the
Three Sisters volcano.
On January 31st, 2022 the US Geological Survey issued a special statement which reported
that the Three Sisters volcano was showing minor signs of unrest, due to a magmatic intrusion
at a depth of approximately 6 kilometers or 4 miles.
It is now nearly 14 months later and this magmatic intrusion is still ongoing, slowly
creeping into the crust, thus displacing rock above it and uplifting overlying rock, creating
a sort of bulge visible to scientific equipment.
Alongside this uplift which has totaled approximately two inches from November of 2021 to March
of 2023, occasional low magnitude earthquakes have also occurred.
Further proving that magma is present at depth is the fact that elevated levels of chloride
have been found in streams and springs close to the uplift center.
Whereas background levels of 0.2 to 0.5 milligrams per liter would be expected, values of 0.6
to 18.6 milligrams per liter were detected.
This magma intrusion has been ongoing since at least 1995, although this date merely details
when it was first detected as it may have been ongoing at a low intensity for more than
a decade beforehand.
This intrusion has peaked and waned in intensity over time but has remained largely constant,
and even produced at least one true earthquake swarm in 2004.
The Three Sisters volcano consists of three separate stratovolcanoes is a north to south
line in addition to flank cinder cones and lava domes.
This volcano is still technically active, although it has not erupted for between 1200
years and 2000 years according to differing sources.
Here is a quick overview of the eruptions it produced during the last 10,000 years showing
eruption vents and lava flows, but you might note that the summits of the three stratovolcanoes
all did not erupt during this time.
The moderately explosive eruptions produced cinder cones and runny lava flows which traveled
up to 7.1 miles distant during this time while viscous rhyolite lava domes created highly
explosive eruptions where individual lava flows traveling only up to 1.4 miles distant
from a source vent.
Yet, in additional to basalt and rhyolite, the three stratovolcanoes are composed of
andesite and dacite.
Although this is not always the case, generally speaking the more silica rich lava involved
in an eruption is, the more explosive that eruption will be.
Thus, knowing what type of magma is intruding underneath the Three Sisters volcano can help
determine what a future eruption might look like.
Therefor, a rhyolite or dacite intrusion would be more potentially concerning in the long
term than a basalt intrusion.
Well, the good news is that the ongoing magmatic intrusion is with a high certainty basaltic
in composition.
This means that if an eventual eruption was to occur due to this intrusion, a new cinder
cone vent would be expected to form above the uplift center, and while this process
of forming the cone would produce moderate amounts of ash it would also produce tall
lava fountains and a several mile long lava flow which would probably travel into a valley
to the west southwest.
We can assume that the intrusion is basaltic for two reasons.
For one, the inflation west of the South Sister stratovolcano has seen only minimal seismic
activity, which once again suggests a fairly hot heat source.
Since rhyolite generally is 850 degrees Celsius and basalt is generally around 1200 degrees
Celsius, basalt seems more likely.
And, according to the U.S. Geological Survey the depth of the intrusion on its own suggests
that it is basalt.
I want to note that the Three Sisters volcano is still at an alert level of green, as although
a magmatic intrusion is ongoing as it has been for 28 years straight an eruption in
my opinion does not appear likely anytime soon at the Three Sisters complex.
My reasoning is that in comparison to other volcanoes showing signs of unrest, the unrest
at Three Sisters in my opinion would be considered fairly weak and the associated magmatic intrusion
probably will not reach the surface in the next decade.
As a final note, here is a general hazard map of the Three Sisters volcano made by the
U.S. Geological Survey which displays the expected radius select but not all volcanic
hazards could reach if a volcanic eruption were to occur.
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